LineArt: Expose the allow_duplication property for instanced objects.

It was missing from the UI.

source/blender/gpencil_modifiers/intern/MOD_gpencillineart.c

@@ -288,6 +288,7 @@ static void panel_draw(const bContext *UNUSED(C), Panel *panel)
           0,
           IFACE_("Overlapping Edges As Contour"),
           ICON_NONE);
+  uiItemR(layout, ptr, "allow_duplication", 0, NULL, ICON_NONE);
 
   gpencil_modifier_panel_end(layout, ptr);
 }